1. Field of the Invention
The present invention relates to a digital signal magnetic recording/reproducing apparatus for recording and reproducing digital signals on a magnetic recording medium such as a video tape recorder (DVTR) or the like.
2. Description of the Prior Art
In conventional digital signal magnetic recording/reproducing apparatuses, due to the difficulty of recording/reproducing the direct current component, baseband modulation techniques having a lower direct current component in the recording signals have been used, such as NRZ codes (e.g., J. K. R. Heitmann "An Analytical Approach to the Standardization of Digital Video Tape Recorder", SMPTE J., 91.3, March 1982, or J. K. R. Heitmann "Digital Video Recording, New Result in Channel Coding and Error Protection", SMPTE J., 93:140-144, February 1984), 8-10 block codes (e.g., J. L. E. Baldwin "Digital Television Recording with Low Tape Consumption", SMPTE J., 88:490-492, July, 1979), Miller-square (M2) codes (e.g., L. Gallo "Signal System Design for a Digital Video Recording System", SMPTE J., 86:749-756, October 1977), ternary partial response system, etc.
However, in the digital signal magnetic recording/reproducing apparatuses utilizing such baseband modulation techniques, due to the basic system of recording the binary signals, the utilization efficiency of the recording frequency band (transmissible bit rate per unit band) is low. For instance, assuming the roll-off rate of the nyquist transmission system to be 0.5, the utilization efficiency of the frequency band is at most 1.33 bits/sec./Hz. This incurs an increased tape consumption making it difficult to record for long time periods. Furthermore, in order to increase the recording rate, a method of expanding the recording frequency band, a method of increasing the number of recording channels, or a method of increasing the relative velocity have been considered. However, if the recording frequency band is expanded, the S/N ratio is greatly deteriorated. Accordingly, the recording rate cannot be increased very much. When the number of recording channels is increased, the track width is narrowed and the S/N ratio is deteriorated. If the relative velocity is increased, the amount of tape consumption naturally comes into question.
To the deterioration of the S/N ratio, the error rate may be improved by using the error control coding. However, the bit rate of the data which can be actually recorded is lost by such coding (Ref. L. M. H. E. Dreissen et al., "An Experimental Digital Video Recording System", IEEE Conf. June 1986, or C. Yamamitsu et al., "An Experimental Digital VTR Capable of 12-hour Recording", IEEE Trans. on CE, CE-33, No. 3, pp 240-248, 1987).
On the other hand, there has been proposed a coded modulation scheme made by combining a multi-value modulation scheme, which shows a high utilization efficiency of the frequency band, with an error control coding for application to digital data transmission. This, however, does not give consideration to the special conditions such as the effects of non-linear distortion or the effects of saturation characteristics in the case of recording/reproducing on a magnetic recording medium (cf. G. Ungerboeck "Channel Coding with Multilevel/Phase Signals", IEEE Trans. on IT, IT-28, No. 1, pp. 55-67, 1982).
The present invention have already invented, as shown in U.S. patent application Ser. No. 07/251,094 (Sept. 29, 1988), a digital magnetic recording/reproducing apparatus using a multi-value modulation scheme which shows high frequency utilization efficiency and high recording rate. However, such apparatus gives no consideration on the effect of the amplitude non-linear characteristic which occurs in the process of magnetic recording and reproducing.